1. Field of the Invention
The present invention is directed to reducing in-band intermodulation products of a television amplification signal.
2. Discussion of the Background
In the transmission of television signals, transmitters typically emit signals of several frequencies at the same time, see for example FIGS. 1(a) and 1(b), such as a visual-carrier frequency, a color sub-carrier frequency and various sideband frequencies (vision content) as well as one or two sound-carrier frequencies. If the visual and sound carrier signals are amplified together, intermodulation between the individual frequencies produces additional sum and difference frequencies. Although these interfering frequencies have the same origins, intermodulation (IM) products are .generated.
Such intermodulation products occur when there is a joint amplification of the visual and sound carrier signals in a non-linear amplifier. This operation is shown in FIGS. 1(a) and 1(b) which show intermodulation (IM) products being formed about the visual carrier and sound carrier in a television transmission channel which has a Sound carrier, color carrier and visual carrier. As shown in FIG. 1(a), the IM products formed about the visual carrier are formed on both sides of the visual carrier (plus and minus) and are separated from the visual carrier by a frequency equal to the absolute value of the difference in frequency between the sound carrier and color carrier. As a result, if the color carrier (in a test function) decreases in frequency (moves to the left in FIG. 1(a)), the frequency of the IM products will move away from the visual carrier, as represented by the arrows in FIG. 1(a). As one example, the visual carrier will typically be at a frequency of 471.25 MHz, the color carrier at a frequency of 474.83 MHz, and the sound carrier at a frequency of 475.75 MHz, and therefore the resulting IM products will be formed at frequencies of 0.92 MHz above and below the visual carrier.
Further, as is shown in FIG. 1(b), when a visual carrier is modulated by modulating signals, IM products are also formed in the area of the sound carrier, only the lower such IM product being shown in FIG. 1(b). This IM product formed in the area of the sound carrier will be a result of an interaction between the modulating signals and the visual carrier. This IM product formed in the area of the sound carrier will be separated from the sound carrier by a frequency equal to the absolute value of the difference in frequency between the visual carrier and the modulating signal. For example, if the visual carrier is at a frequency of 471.25 MHz, the sound carrier is at a frequency of 475.75 MHz, and the modulating signals are at frequencies of 0.5 MHz above and below visual carrier, then the IM product shown in FIG. 1(b) will be at a frequency of 475.25 MHz, which is 0.5 MHz below the sound carrier.
In this way, when any combination of multiple carriers is amplified in a non-linear device, various intermodulation products are developed. In the case of common amplification of a complete television signal package (visual, color and sound carriers and modulation sidebands), intermodulation products appearing both within band and out-of-band are produced.
It is known how to remove out-of-band intermodulation products. To remove such out-of-band intermodulation products, notch filters or bandpass structures can be used to limit the energy levels of such out-of-band intermodulation products.
However, such conventional filtering cannot work to effectively remove the in-band intermodulation products as the in-band intermodulation products appear in a spectrum that also contains significant side band energy developed in the normal modulation process and which is needed by a television receiver for complete video demodulation.
Therefore, dealing with in-band intermodulation products is much more difficult. More particularly, in-band intermodulation products cannot be limited by conventional filtering methods. Conventionally, the correction of in-band intermodulation products is accomplished by circuitry that provides effective correction for intermodulation products (pre-distortion) close to the visual carrier only, i.e., without regard to the higher products. Such conventional intermodulation correction circuitry is deficient in that it does not effectively correct for intermodulation in-band products at higher frequency (video) areas of the passband because those products can have different phase characteristics than the pre-correction products as a result of phase non-linearities in the amplifiers. Such conventional intermodulation correction circuitry uses low frequency linearity and incidental carrier phase modulation (ICPM) techniques.
More particularly, conventional techniques for IM reduction are to pre-distort the low level signal package through a controlled non-linearity. This technique creates IM products equal in amplitude, but Opposite in phase, i.e. 180.degree. out of phase, to those created in the high power amplifier. Thus, the offending IM products are cancelled out.
Conventional IM cancellation techniques rely on the fact that the IM products are created by fixed frequency signals and, thus, always appear at the same frequencies. That is, since the IM products are formed by an interaction of the visual, color and sound carriers, which are always at the same known frequencies, the IM products will also always appear at the same known frequencies. To cancel such IM products then, complementary IM cancellation products, i.e. IM products equal in amplitude and of opposite phase, are formed at the known frequencies. This technique can be called narrow band IM correction and it is the technology used by most common amplification transmitters in manufacture today.
In testing for the cancellation IM products, one can vary the frequency of the color carrier across the full signal bandwidth, resulting in the corresponding IM products also changing frequency, as shown by the arrows in FIG. 1(a). This varying color carrier effectively generates the in-band IM cancellation products out of phase with the generated IM products in certain regions. However, the phase of the IM products across the band will vary as a result of phase non-linearities in the high power amplifiers used prior to transmission of the television signals. Thus, the carefully created cancellation ratios between amplitude and phase become subject to the variations in the amplifier's phase and gain response across the band. This tends to destroy the IM cancellation and results in the situation shown in FIG. 2. FIG. 2 shows amplitudes (locus) of IM products across a television signal bandwidth. As Shown in FIG. 2, IM products will be generated at a higher amplitude in the region of the color and sound carriers. These IM products generated in the region of the color and sound carriers are not effectively cancelled as a result of their amplitudes. In fact, with enough phase shift the IM products can become enhanced.